EUV lithography (EUVL) is an emerging technology in the microelectronics industry. It is one of the leading candidates for the fabrication of devices with feature sizes of 70 nm and smaller. Synchrotron radiation facilities provide a convenient source of EUV radiation for the development of this technology. This invention relates to techniques for generating arbitrary fill patterns that simulate actual fill patterns for potential stepper designs, or generate specialized fill patterns for more general optical processing systems.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a cast image of the subject pattern. Once the image is cast, it is indelibly formed in the coating. The recorded image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent or opaque to the impinging radiation. Exposure of the coating through the transparency placed in close longitudinal proximity to the coating causes the exposed area of the coating to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing as described in the previous paragraph. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of 10 to 20 nm) are now at the forefront of research in efforts to achieve smaller transferred feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection (demagnifying) lens onto a wafer. Reticles for EUV projection lithography typically comprise a glass substrate coated with an EUV reflective material and an optical pattern fabricated from an EUV absorbing material covering portions of the reflective surface. In operation, EUV radiation from the illumination system (condenser) is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation is re-imaged to the wafer using a reflective optical system and the pattern from the reticle is effectively transcribed to the wafer.
A source of EUV radiation is the laser-produced plasma EUV source, which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000 C, to emit EUV radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line so that malfunction does not close down the entire plant. A stepper employing a laser-produced plasma source is relatively inexpensive and could be housed in existing facilities. It is expected that EUV sources suitable for photolithography that provide bright, incoherent EUV radiation and that employ physics quite different from that of the laser-produced plasma source will be developed. One such source under development is the EUV discharge source.
EUV lithography machines for producing integrated circuit components are described for example in Tichenor et al. U.S. Pat. No. 6,031,598. Referring to FIG. 11, the EUV lithography machine comprises a main vacuum or projection chamber 2 and a source vacuum chamber 4. Source chamber 4 is connected to main chamber 2 through an airlock valve (not shown) which permits either chamber to be accessed without venting or contaminating the environment of the other chamber. Typically, a laser beam 30 is directed by turning mirror 32 into the source chamber 4. A high density gas, such as xenon, is injected into the plasma generator 36 through gas supply 34 and the interaction of the laser beam 30, and gas supply 34 creates a plasma giving off the illumination used in EUV lithography. The EUV radiation is collected by segmented collector 38, that collects about 30% of the available EUV light, and directed toward the pupil optics 42. The pupil optics consists of long narrow mirrors arranged to focus the rays from the collector at grazing angles onto an imaging mirror 43 that redirects the illumination beam through filter/window 44. Filter 44 passes only the desired EUV wavelengths and excludes scattered laser beam light in chamber 4. The illumination beam is then reflected from the relay optics 46, another grazing angle mirror, and then illuminates the pattern on the reticle 48. Mirrors 38, 42, 43, and 46 together comprise the complete illumination system or condenser. The reflected pattern from the reticle 48 then passes through the projection optics 50 which reduces the image size to that desired for printing on the wafer. After exiting the projection optics 50, the beam passes through vacuum window 52. The beam then prints its pattern on wafer 54.
Although no longer under serious consideration for high-volume commercial fabrication applications, synchrotron sources play an extremely important role in the development of EUV lithography technology. Being readily available, highly reliable, and efficient producers of EUV radiation, synchrotron radiation sources are well suited to the development of EUV lithography. These sources are currently used for a variety of at-wavelength EUV metrologies such as reflectometry, interferometry, and scatterometry.
In the case of synchrotron radiation sources, there are three types of sources: bending magnets, wigglers, and undulators. In bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted. Wiggler sources comprise a so-called wiggler for the deflection of the electron or of an electron beam. The wiggler includes a multiple number of alternating poled pairs of magnets arranged in a series. When an electron passes through a wiggler, the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further characterized by the fact that no interference effects occur. The synchrotron radiation produced by a wiggler is similar to that of a bending magnet and radiates in a horizontal steradian. In contrast to the bending magnet, it has a flux that is reinforced by the number of poles of the wiggler.
Finally, in the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and a smaller magnetic field of the deflection pole than in the case of the wiggler, so that interference effects of synchrotron radiation occur. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and radiates both horizontally and vertically in a small steradian element, i.e., the radiation is strongly directed.
In lithographic applications, the partial coherence of the illumination (sigma) is often defined as the ratio of the illumination angular range to the numerical aperture of the imaging (projection optical) system. The illumination angular range is also referred to as the divergence of the source. Undulator radiation is much like a laser source in that it produces highly-coherent light of very low divergence. A typical EUV undulator beamline produces a sigma of less than 0.1 whereas lithographic application nominally require a sigma of 0.7 or higher. Although less coherent than undulator radiation, bending magnet radiation is also typically too coherent to be directly used for lithography.
Currently the coherence and high flux properties of synchrotron undulator radiation are being used for crucial at-wavelength interferometry and alignment of complex EUV lithography optics. These interferometry results can be used to predict imaging performance, however, the final performance metric must always be actual imaging. In addition, the properties of the illuminator (condenser) play an important role in the final imaging performance. To study the effectiveness of various illuminator designs, it would be beneficial to have an illuminator simulator that generates arbitrary pupil-fill patterns and that affords a simple transition between theses patterns. Such a device is nearly impossible to achieve using conventional illuminators comprised of numerous, complicated optics; thus, novel-illuminator performance studies are typically limited to computer simulations.
The function of a lithographic illuminator is essentially to illuminate the reticle to be imaged with a range of angles. The present invention is based, in part, on the recognition that the illumination can be employed to generate a pattern in the pupil of the imaging system, where spatial coordinates in the pupil plane correspond to illumination angle in the reticle plane. Specifically, a coherent synchrotron beamline is used along with a potentially decoherentizing holographic optical element (HOE), as an experimental EUV illuminator simulation station. The pupil fill is completely defined by a single HOE, thus the system can be easily modified to model a variety of illuminator fill patterns. Also the HOE allows the generated pupil fill pattern to be arbitrary. Because a HOE can be designed to generate any desired angular spectrum such a device can serve as the basis for an arbitrary illuminator simulator. It should be noted that a valid simulation of the pupil fill effects also requires the individual angles produced by the HOE to be mutually incoherent. Achieving this condition requires that the HOE be moved relative to the stationary, coherent, illumination beam such that many thousand correlation lengths are covered over the total image integration (exposure) time.
The HOE can be a transmission or reflection device and can be readily implemented using a binary phase or amplitude carrier. At EUV wavelengths, the preferred embodiment uses a binary phase carrier HOE generated by fabricating a binary relief structure onto a smooth substrate and overcoating the device with a conventional reflective EUV multilayer. Positioning the HOE such that it coincided with an existing mirror optimizes the efficiency of the system. In the preferred embodiment, the HOE serves as the effective source in a critical illumination system. The HOE is re-imaged to the reticle by way of a spatial filter system that removes all but the desired holographic order. In this case the reticle plane illumination is set by the illumination of the HOE itself and the pupil fill is set by the far-field diffraction of the HOE. It is also possible to use the HOE in a Kxc3x6hler configuration where the reticle illumination pattern is set by the HOE diffraction pattern and the pupil fill is set by the HOE illumination pattern.
As with any illuminator relying on a passive element to reduce the coherence of a coherent beam, the HOE must be moved at a rate fast relative to the observation (exposure) time in order for the desired coherence modification to be achieved. Without motion, the HOE creates the requisite multiple angles of illumination, however, the light at each of these illumination angles remains mutually coherent as they are all derived from a single coherent beam. Incoherence requires both multiple angles of incidence and mutual incoherence of all these angles. This can be effectively achieved by rapid motion of the HOE. The HOE, however, cannot simply be rotated as is typically done with conventional diffusers because this would cause the carrier to rotate in space. Instead, the HOE must be translated in x and y only.
In one embodiment, the invention is directed to an illuminator device, for an optical image processing system, wherein the image processing system comprises an optical system that employs an arbitrary pupil fill pattern, and wherein the illuminator device includes:
a source of coherent or partially coherent radiation;
a holographic optical element (HOE) having a surface that receives incident radiation from said source; and
a condenser optic that re-images a surface of the holographic optical element to the entrance of said image processing system.
In another embodiment, the invention is directed to a method of generating an arbitrary pupil fill pattern in the entrance of an image processing system that includes the steps of:
(a) directing incident radiation onto a surface of a holographic optical element (HOE) wherein the radiation is coherent or partially coherent radiation; and
(b) re-imaging the surface of the HOE to the entrance of the image process system.